(1) Field of the Invention
This invention relates to a die assembly for extruding multilayer parisons which parisons are used for subsequent blow molding of bottles or the like. More particularly, the invention relates to a die assembly for extruding multilayer parisons which comprises a plurality of layers of thermoplastic synthetic resins.
Furthermore, the present invention relates to the die assembly for extruding multilayer parisons in which different kinds of thermoplastic resins are thermally joined together in layers, thereby facilitating to produce blow molded articles that have several advantageous properties particular to the respective resin materials used.
(2) Description of the Prior Art
Blow molded articles have hitherto been widely used as containers for foodstuffs, chemicals, medicines, cosmetics, detergents and so forth. They have been, however, generally of a single layer structure. As the raw materials for the blow molded articles, thermoplastic resins such as polyethylene, polypropylene, polycarbonate, polystyrene and polyvinyl chloride are commonly employed.
In view of the public concern in recent years such as the saving of oil and electric resources, the disposal of waste plastic materials and the harmful effect of plastics in view of food sanitation especially in connection with the plasticizer contained in polyvinyl chloride products, it is required that the containers or bottles must be produced with possible minimum quantities of materials and, at the same time, the containers must satisfy the requirements in use and have no problem in view of environmental pollution and food sanitation. Nevertheless, the ordinary monolayer blow molded products are not able to satisfy simultaneously all the above-mentioned requirements, especially cannot meet the requirements for chemical stability, gas barrier property, and other properties of container walls.
Taking the laminated film materials into consideration, there are proposed various devices to introduce laminated structure into blow molding technique. That is, several properties of thermoplastic resins such as hardness, flexibility, tensile strength, gas permeability, chemical resistance and printability are different with the kinds of resins. While the outer surface of a bottle is liable to receive various external effects and influences and the appearance of the bottle is mainly influenced by the kind of material used. Furthermore, when a bottle is to be printed, the outer material must have affinity to printing inks. On the other hand, the inside surface of a plastic bottle comes into direct contact with the material contained therein, so that the inside surface of the bottle must neither react with the contents nor release any extraneous substance into the contents.
In view of the above facts, when plastic bottles are made double layer structure or multiple layer structure and most suitable materials are selected for respective layers, more desirable bottles can be produced from smaller amount of resin materials as compared with the conventional single layer bottles.
A proposal for such a purpose is described in U.S. Pat. No. 4,047,868, "Multilayer Parison Extrusion Molding Machine for Blow Molding". In this machine, it is possible to produce triple layer parison. However, when an outer layer and an inner layer are made by the resins which cannot be well joined together, the intermediate layer must be used for an adhesive material. In such a case, only double layer parisons are formed. Furthermore, it is not possible to produce parisons having more than three layers. Still further, due to the structure of die head, assembling and disassembling, and controlling of resin quantity are difficult and the leakage of resin is liable to occur during extrusion of parisons.
Meanwhile, in usual practice of plastic molding, it is necessary to use recycled material without discarding condemned goods and flashes or fins in order to reduce production costs. Therefore, parisons are composed of, for example:
a. Inner layer that is chemically resistant, innocuous, odorless, etc. PA1 b. Adhesive layer PA1 c. Gas-barrier layer PA1 d. Adhesive layer PA1 e. Recycled layer PA1 f. Outer layer that is shock resistant, scratch resistant, glossy, printable, etc.
In some case, a layer or layers having other characteristic properties may be interposed between the above layer materials to form parisons of five or more layer structure.